Solder-precipitating composition and mounting method using the composition

ABSTRACT

The invention provides a solder-precipitating composition containing Sn-Pb alloy powder and an organic acid salt of Pb as essential components, and capable of precipitating solder as a result of substitution between Sn contained in the Sn-Pb alloy powder and Pb contained in the organic acid salt of Pb. Further, the invention provides a mounting method including the steps of supplying a conductor on a board with a solder-precipitating composition which contains Sn-Pb alloy powder and an organic acid salt of Pb as essential components, heating the solder-precipitating composition supplied on the conductor, to precipitate solder on the conductor and pre-coat the conductor with solder, as a result of substitution reaction between Sn in the Sn-Pb alloy powder and Pb ions in the organic acid salt of Pb, mounting a device on the solder pre-coated conductor, and melting the solder layer to securely mount the device on the conductor.

TECHNICAL FIELD

This invention relates to a solder-precipitating composition and amounting method using the composition, and more particularly to acomposition and a mounting method using the composition, which arecapable of forming, in a relatively short time, an uniform andrelatively-thick solder layer on each of conductive portions (pads) of acircuit board, and pre-coating solder without creating a solder bridgeeven when the distance between the conductive portions is narrow.

BACKGROUND ART

A conventional solder composition (solder paste) used in a surfacemounting technique is in the form of a paste which is obtained bydispersing, in a flux, solder powder as Sn-Pb alloy, and mixingnecessary additives therein.

To mount an electronic device on a circuit board using a soldercomposition, the solder composition is put on a pad which serves as amounting portion, and the device is placed on the board such that theterminal of the device is positioned on the solder composition. Then,the resultant structure is heated to melt the solder composition andelectrically connect the device to the pad.

In the above structure, the device can be electrically connected to thepad in a reliable manner if they are sufficiently separated from eachother. However, where they cannot sufficiently be separated in ahigh-density mounting, it is possible that portions of solder areconnected to each other at the time of putting the solder composition onthe pad or melting the same, thereby forming a solder bridge. When thesolder bridge has been formed, short-circuiting will occur whichdegrades the circuit function. Moreover, repairing the solder bridgerequires high cost.

As a solder-precipitating composition free from the above problem, U.S.Pat. No. 5,145,532 (Jpn. Pat. Appln. KOKAI Publication No. 1-157796)discloses a solder-precipitating composition which contains Sn powderand an organic acid salt of Pb.

This solder-precipitating composition has the following solderdeposition mechanism:

1) The organic acid salt of Pb is separated into organic acid ions andPb ions by heating;

2) Pb ions are displaced with nonionic Sn which has an ionizationtendency higher than Pb, with the result that Pb ions are reduced tononionic Pb and part of nonionic Sn is oxidized to Sn ions; and

3) The reduced Pb is dispersed into the other part of Sn (i.e. itsnonionic portion), thereby forming a solder.

It was found that the above-described solder precipitation process showsbehaviors which significantly differs from those shown in the case ofmelting solder at its melting point. For example, in the case ofpre-coating pads of a printed circuit board with solder, using theconventional solder composition, adjacent portions of solder paste maybe connected to each other after heating unless an appropriate amount ofsolder paste which is proportional to the area of the pad is supplied,thus forming a solder bridge. On the other hand, in the case of thesolder-precipitating composition disclosed in U.S. Pat. No. 5,145,532,it is not necessary to supply the solder-precipitating composition bythe printing method using a stencil corresponding to a pad pattern. Thissolder-precipitating composition is characterized in that even if thesolder-precipitating composition is supplied on the overall surface ofthe board, solder is coated only on conductive pads, and no solderbridges are formed. This feature has now been highlighted under thecircumstances that high-density circuit boards have been developed inaccordance with downsizing of electronic parts. In such high-densitycircuit boards, it is extremely difficult to supply a solder compositionby the printing method so as to avoid occurrence of solder bridges. Inthe solder-precipitating composition of U.S. Pat. No. 5,145,532,however, the above-described solder precipitation mechanism exerts oneach Sn particle. It has been found that in Sn powder having a certainparticle size distribution, the smaller the diameter of a particle andthe higher the surface area ratio of the same, the higher the reactionspeed (in other words, the larger the diameter, the lower the reactionspeed). In light of this, the solder-precipitating composition of U.S.Pat. No. 5,145,532 inevitably shows variations in a final or targetsolder composition. This means that it is difficult to perform uniformpre-coating of solder. Sn powder of a uniform particle size may wellcause uniform reaction. However, preparation of such Sn powder costs toomuch.

Moreover, in the case of obtaining an alloy consisting of 63 wt % Sn and37 wt % Pb only by displacement reaction between Sn powder and Pb ionsin the organic acid salt of Pb, the amounts of those portions ofnonionic Sn and the organic acid salt of Pb, which are used only in ionexchange reaction and are not finally converted into a solder alloy,increase in the solder-precipitating composition. Accordingly, theamounts of effective components decrease in the solder-precipitatingcomposition.

As aforementioned, in the conventional soldering method for soldering anelectronic device, a solder composition is supplied on the pads of aprinted circuit board by the printing method, and then the electronicdevice is mounted on the solder composition. Thereafter, the soldercomposition is made to reflow to solder the lead portions of the device.In addition to this soldering method, a method is put to practice, inwhich an electronic device is mounted after solder is pre-coated on thepads of a board and then a flux is supplied thereon, thereby heating thepre-coated solder to make it to reflow and fix the lead portions of thedevice. At the time of mounting an LSI electronic device such as QFP(Quad Flat Package), it is necessary to pre-coat solder such that thepre-coated solder layer has a thickness sufficient to absorb variationsin lead coplanality between the leads of the component, in order toreliably fix the leads to the pads provided on the same surface of theboard.

However, in the case of pre-coating the pads of a printed circuit boardwith a great amount of solder, using the solder-precipitatingcomposition of U.S. Pat. No. 5,145,532, it is necessary to use a thickmask which makes printing control difficult, to supply thesolder-precipitating composition on the board by the printing method asa general method. Furthermore, even if a great amount ofsolder-precipitating composition is supplied to form a thick solderlayer on the board, the solder-precipitating composition inevitablymelts and flows out during heating, with the result that the pads cannotbe pre-coated with a great amount of solder. Therefore, it is necessaryto repeatedly supply the solder-precipitating composition on the boardand repeatedly subject it to substitution reaction.

DISCLOSURE OF INVENTION

The invention has been developed under the above-describedcircumstances, and aims to provide a solder-precipitating compositioncapable of pre-coating pads with a solder layer of a uniform andsufficient thickness in a short time, and capable of pre-coating thepads with solder without solder bridges even if the pads are arranged atnarrow intervals.

The invention provides a solder-precipitating composition containingSn-Pb alloy powder and an organic acid salt of Pb as essentialcomponents, and capable of precipitating solder as a result of ionexchange between Sn contained in the Sn-Pb alloy powder and Pb containedin the organic acid salt of Pb.

Further, the invention provides a mounting method comprising the stepsof: supplying a conductor on a board with a solder-precipitatingcomposition which contains Sn-Pb alloy powder and an organic acid saltof Pb as essential components; heating the solder-precipitatingcomposition supplied on the conductor, to deposit solder on theconductive member and pre-coat the conductive member with solder, as aresult of substitution reaction between Sn in the Sn-Pb alloy powder andPb ions in the organic acid salt of Pb; mounting a device on the solderpre-coated conductor; and melting the solder layer to securely mount thedevice on the conductor.

BRIEF DESCRIPTION OF DRAWING

The drawing is a graph, showing the relationship between the heatingtime period and the Sn content of solder.

BEST MODE FOR CARRYING OUT INVENTION

A solder-precipitating composition according to the invention ischaracterized in that it contains Sn-Pb alloy powder and an organic acidsalt of Pb.

In the solder-precipitating composition, ion exchange reaction between adevice (Sn), which is contained in Sn-Pb alloy powder and has highionization tendency, and Pb contained in the organic acid salt of Pbadvances on the surfaces of the Sn-Pb alloy particles dispersed in thecomposition. In other words, part of Sn is ionized, while Pb ions in theorganic acid salt of Pb are converted to nonionic Pb. The thus-obtainedPb encounters the Sn-Pb alloy, thereby forming an Sn-Pb alloy containingthe encountered Pb. While Pb is alloyed in the Sn-Pb alloy, the alloy isshifted from an Sn-rich state to a Pb-rich state. Accordingly, themelting point of the Sn-Pb alloy is reduced, and a surface portion ofthe alloy starts to melt, thereby coating each pad with solder. On thesurface of the pad, solder is deposited by means of an activator in thesolder-precipitating composition.

Since the mechanism of the invention is based on atom-level alloying,uniform reaction occurs between the Sn-Pb alloy particles and theorganic acid salt of Pb in the solder-precipitating composition.Therefore, the Sn-Pb alloy particles do not agglomerate into largeparticles, and solder is gradually deposited on the pads with theparticle-dispersed state of the solder-precipitating compositionunchanged. As a result, solder is coated without solder bridges. Sincethe invention has this mechanism, an appropriate solder layer is formedon each pad even if the pads are arranged at small intervals, andoccurrence of a solder bridge is greatly restrained.

In the above-described mechanism, the speed of deposition of solder(alloying reaction) is determined on the basis of the number ofoccasions of atom-level collision of Sn and Pb. Since thesolder-precipitating composition of the invention contains an alloypowder, the reaction time required until a final solder composition isobtained is shortened. In other words, since in the case of using thealloy powder, the precipitation of solder does not depend only upon thecollision of a metal powder against a nonionic metal obtained as aresult of substitution reaction, the amount of displaced metal obtainedby ion exchange reaction can be reduced as compared with the case ofusing a single-metal powder. Since the amount of substituted metalobtained by substitution reaction can be reduced with the use of analloy powder, the amount of metal contained in the solder-precipitatingcomposition can be increased, which facilitates coating of arelatively-thick solder layer. In addition, variations in soldercomposition can be reduced since they substantially reflect variationsin the composition of an alloy powder used.

Moreover, the use of an alloy powder can increase the metal ratio of thesolder-precipitating composition, and also increase the amount ofeffective components for coating each pad of a circuit board withsolder. On the other hand, in the case of the solder-precipitatingcomposition disclosed in U.S. Pat. No. 5,145,532, in order to form anSn-Pb eutectic structure only of nonionic Sn, a thick mask is necessaryto supply a great amount of solder-precipitating composition by theprinting method at the time of coating pads with solder. This is becausethe ratio of that portion of Sn powder and the organic acid salt of Pb,which is used only in substitution reaction, to the overallsolder-precipitating composition is high; in other words, thesolder-precipitating composition contains only a small amount ofeffective components for forming solder.

In this case, it is difficult to control the thickness of coating of thesolder-precipitating composition, and the efficiency of cleaningperformed in a later process is degraded. Further, even if a thicksolder-precipitating composition layer is supplied on a circuit board,it flows out and becomes thinner during heating. Thus, the compositionis not all pre-coated. In other words, the amount of solder pre-coatedon a pad is limited. This being so, to pre-coat the pad with a thicksolder layer, it is necessary to repeat the processes for supplying thesolder-precipitating composition, subjecting the same to reaction andcleaning the resultant structure. Thus, the solder-precipitatingcomposition of U.S. Pat. No. 5,145,532 has a very low coatingefficiency.

In the case of using the solder-precipitating composition according tothe invention, however, a relatively thin mask whose thickness is easyto control can be used since the composition has effective metalcontents, and the cleaning can be performed in an easy manner in a laterprocess. To pre-coat a thick solder layer, it suffices if each of theprocesses for supplying the solder-precipitating composition, subjectingthe same to reaction and cleaning the resultant structure is performedonly one time.

In the invention, to obtain a eutectic alloy of Sn-Pb (Sn/Pb=63/37) asthe final solder composition, Sn-Pb alloy powder having an Sn ratiohigher than 63/37 is used. Moreover, to obtain a Pb-rich soldercomposition as the final solder composition, Sn-Pb alloy powder havingan Sn content higher than the final solder composition is used. Toobtain a final solder composition having an Sn/Pb ratio of 10/90, Sn-Pballoy powder having an Sn content higher than Sn/Pb=10/90 is used as thesolder-precipitating composition. The above-described Sn-Pb alloys maycontain a small amount of some other metal such as Ag or Cu.

It is preferable in the invention to set the diameter of each alloyparticle to 1-20 μm. This is because if the diameter is less than 1 μm,the surface of alloy is oxidized and only a small amount of solder isprecipitated. Further, if the diameter exceeds 20 μm, the range ofvariations in the precipitation amount of solder is wide.

It is preferable in the invention to set the amount of an alloy powderin the solder-precipitating composition to 10-90 wt %. This is becauseif the amount of the alloy powder is less than 10 wt %, a sufficientamount of solder cannot be precipitated. If the amount exceeds 90 wt %,it is difficult to keep the solder-precipitating composition in a pastestate. Most preferably, the amount of the alloy powder is 20-70 wt %.

An organic acid constituting the organic acid salt of Pb consists, forexample, of a rosin or its derivative; an aliphatic carboxylic acid suchas stearic acid, oleic acid, neodecanoic acid, sebacic acid, fumaricacid, etc.; an aromatic carboxylic acid such as benzoic acid, phthalicacid, isophthalic acid, trimellitic acid, pyromellitic acid, etc.; ornaphthenic acid, etc. Further, a polybasic acid such as a dibasic acid,a thribasic acid or the like may be used.

In particular, it is preferable to use, as the organic acid, a rosin orits derivative, or naphthenic acid. As the rosin, gum rosin, tall oilrosin, wood rosin, etc. is used. Each of pure substances such as ableticacid, pimaric acid, etc., which are main components of rosin, may beused in place of the rosin. As the derivative of the rosin,disproportionated rosin, hydroten-added rosin, maleic or fumaric rosin,etc. may be used. The kind of the organic acid salt of Pb is selected inaccordance with solder pre-coating conditions (temperature, atmosphere,etc.).

In the solder-precipitating composition of the invention, Ag or Cu maybe contained as a third component in addition to Sn and Pb by adding asmall amount of an organic acid salt of Ag or an organic acid salt ofCu.

It is preferable to set the content of the organic acid salt of Pb inthe solder-precipitating composition to 3-60 wt % (in the case of Pbnaphthenic containing 20 wt % Pb, for example, the content of Pb is0.6-12 wt %). This is because if the content of the organic acid salt ofPb is less than 3 wt %, the boundary tension of a precipitated solder islow and hence a solder bridge is liable to occur. On the other hand, ifthe content exceeds 60 wt %, the paste state of the solder-precipitatingcomposition is unstable and hence cannot be put on pads with accuracy atthe time of pre-coating solder. The most preferable content of theorganic acid salt of Pb is 7.5-50 wt % (in the case of Pb naphthenatecontaining 20 wt % Pb, for example, the content of Pb is 1.5-10 wt %).

It is preferable to set the Pb content of the organic acid salt of Pb to1.5 wt % or more of the solder-precipitating composition. This isbecause if the Pb content is less than 1.5 wt % of thesolder-precipitating composition, the effect of Pb for restraining theoccurrence of a solder bridge at the time of solder pre-coating isreduced. Moreover, it is preferable to set the Pb content of the organicacid salt of Pb to 12 wt % or less of the solder-precipitatingcomposition, since a variation may well occur in solder compositionwhere too much Pb is contained.

The Pb content of the solder-precipitating composition, which caneffectively restrain the occurrence of a solder bridge, depends upon thecomposition of an alloy powder or the kind of an organic acid salt ofPb. Further, adjusting the Pb content of an organic acid salt of Pbenables appropriate pre-coating of solder on a fine pattern. Since atoo-low Pb content of an organic acid salt of Pb inevitably reduces thepattern-following ability of the solder-precipitating composition, it isnecessary to set the Pb content to an allowable value.

In the invention, the ratio between Sn and Pb (the sum of Pb containedin an alloy powder and an organic acid salt of Pb) is determined inaccordance with a solder composition suitable to use. To obtain aeutectic solder composition, the ratio of Sn to Pb is set to 63/37.

If necessary, the solder-precipitating composition of the invention maybe in the form of a paste in which the following agents are added: anadhesive such as gum rosin, etc.; an activator such as diethanolamine,triethanolamine, etc.; a solvent such as butylcarbitol, mineral spirit,etc.; and a viscosity-holding agent such as castor wax, cellulosepowder, etc. In the case of adding these additives in thesolder-precipitating composition, they may be directly mixed with analloy powder and an organic acid salt of Pb. Alternatively, a flux maybe beforehand prepared by mixing the adhesive, the activator and thesolvent, and then mixed with the alloy powder and the organic acid saltof Pb.

To deposit solder using the solder-precipitating composition of theinvention, it is necessary to set the heating temperature for solderprecipitation reaction to a value higher than the solidus line of thesolder. This is because setting the heating temperature to a value lowerthan the solidus line at the time of solder precipitation causes solidphase/solid phase diffusion of Pb precipitated from the organic acidsalt of Pb and the alloy powder in the solder-precipitating composition.As a result, reaction advances at extremely low speed and a great amountof time is required to obtain a final solder composition. On the otherhand, if the heating temperature is set higher than the solidus line,reaction advances at high speed and the final solder composition can beobtained in a short time. Specifically, it is preferable to set theheating temperature to 210°-220° C.

To mount an electronic device on a pad of a printed circuit board withthe use of the solder-precipitating composition of the invention,firstly, the solder-precipitating composition, which contains Sn-Pballoy powder and an organic acid salt of Pb as essential components, issupplied on the pad, and then heated to deposit solder as a result ofsubstitution reaction between nonionic Sn in the Sn-Pb alloy powder andPb ions in the organic acid salt of Pb. Thereafter, the electronicdevice is placed on the pre-coated solder, and fixed to the pad bymelting the solder. To melt the solder pre-coating, the precipitatedsolder is heated to a temperature higher than its melting point.

Examples of the solder-precipitating composition of the invention willnow be explained in detail.

EXAMPLE 1

First, a flux A in the form of paste was prepared by mixing 60 wt % gumrosin as an adhesive, 10 wt % diethanolamine as an activator, 10 wt %castor wax, and 20 wt % butylcarbitol as a solvent.

Subsequently, a solder-precipitating composition according to theinvention was prepared by mixing 40 wt % solder powder (Sn/Pb=70/30) ofan average particle size of 10 μm, 20 wt % Pb naphthenate containing 24wt % Pb, as the organic acid salt of Pb, and 40 wt % flux A.

EXAMPLE 2, COMPARATIVE EXAMPLE 1

An example 2 and a comparative example 1 were prepared in a mannersimilar to the example 1, except for that they contain an alloy powder,Sn powder and Pb naphthenate which have compositions as shown in table1, respectively.

                  TABLE 1                                                         ______________________________________                                        COMPO-   SOLDER    Sn        Pb NAPH- FLUX                                    SITION   POWDER    POWDER    THENATE  A                                       ______________________________________                                        EXAMPLE 1                                                                              40 *1               20       40                                      EXAMPLE 2                                                                              60 *1               20       20                                      COMPARA-           25        45       30                                      TIVE                                                                          EXAMPLE 1                                                                     ______________________________________                                        (UNIT: wt %)                                                                   *1 Sn/Pb = 70/30                                                              *2 Sn/Pb = 25/75                                                         

Each of the solder-precipitating compositions of the examples 1 and 2and the comparative example 1 was coated on a 160-pin QFP pattern (thepitch of the pins is 0.3 mm) formed on a printed circuit board, suchthat the layers of the compositions had a thickness of 250 μm. Theresultant structures were placed on a heating plate of 250° C. for 120seconds, thereby precipitating solders. The QFP pattern was formed of Niplated with Au by flushing. The resultant board was cleaned by means oftoluene, and then the solders were dissolved by an acid into liquidsamples, which were subjected to atomic absorption analysis. Table 2shows the results of the analysis.

                  TABLE 2                                                         ______________________________________                                                     THEORETICAL   ANALYSIS                                           COMPOSITION  COMPOSITION   RESULTS                                            ______________________________________                                        EXAMPLE 1    Sn/Pb = 60/40 Sn/Pb = 62/38                                      EXAMPLE 2    Sn/Pb = 20/80 Sn/Pb = 22/78                                      COMPARATIVE  Sn/Pb = 63/37 Sn/Pb = 66/34                                      EXAMPLE 1                                                                     ______________________________________                                    

As is evident from table 2, solders having compositions close to atheoretical composition are obtained from the examples 1 and 2 and thecomparative example 1 as a result of a heating treatment at 250° C. Thismeans that ion exchange reaction (substitution reaction) in thesolder-precipitating compositions, which is the mechanism of theinvention, occurred. In table 2, the theoretical composition indicates atarget composition for precipitating solder.

Further, with respect to the solder-precipitating compositions of theexamples 1 and 2 and the comparative example 1, variations in soldercomposition between pre-coated solder layers were examined.Specifically, solder layers are coated on 20 QFP patterns with the useof each of the above-described solder-precipitating compositions. Then,20 liquid samples obtained from the 20 solder layers per eachsolder-precipitating composition were subjected to atomic absorptionanalysis to measure their Sn contents. The variations were obtained bycalculating the difference between the maximum value and the minimumvalue of the Sn contents measured per each solder-precipitatingcomposition. The calculation results are shown in table 3. Moreover, thenumber of bridges in the 160 pins of each solder layer coated on the QFPpattern was counted by the eyes. The counted results are also shown intable 3.

Thereafter, the time (treatment time) required until each solder wasprecipitated was measured, with respect to the solder-precipitatingcompositions of the examples 1 and 2 and the comparative example 1.Specifically, four solder-precipitating composition layers of eachcomposition were formed on four QFP patterns, and were heated for 30,60, 90 and 120 seconds, respectively. Liquid samples obtained from theresultant solder layers were subjected to atomic absorption analysis tomeasure their Sn contents. The time required until the Sn content ofeach liquid sample reached 90% or more of the Sn content of a targetsolder composition was deemed as the required treatment time of theliquid sample. The measured required time periods are also shown intable 3.

Subsequently, the thickness of each solder layer was measured, withrespect to the solder-precipitating compositions of the examples 1 and 2and the comparative example 1. Specifically, a 200 μm-thicksolder-precipitating composition layer of each composition was formed oneach QFP pattern, and was heated at 250° C. for 120 seconds to form asolder layer. The thickness of each solder layer was measured by asurface-roughness meter. The measured thicknesses are shown in table 3.

                  TABLE 3                                                         ______________________________________                                                 DISPRE-   REQUIRED                                                            SIONS     SOLDER     SOL-                                                     IN        PRECIPI-   DER    NUMBER                                            SOLDER    TATING     LAYER  OF                                                COMPO-    TIME       THICK- SOLDER                                            SITION    PERIOD     NESS   BRIDGES                                           *3        *4         (μm)                                                                              *5                                       ______________________________________                                        EXAMPLE 1                                                                              2.2       60         37     0                                        EXAMPLE 2                                                                              1.1       60         40     0                                        COMPARA- 8.2       120        24     0                                        TIVE                                                                          EXAMPLE 1                                                                     ______________________________________                                         *3: Difference between the maximum value and the minimum value of the Sn      contents of 20 liquid samples                                                 *4: Time (seconds) required until the Sn content of each liquid sample        reaches 90% or more of the Sn content of a target solder composition          *5: The number of bridges in the QFP 160 pins                            

As is evident from table 3, with respect to the solder-precipitatingcompositions of the examples 1 and 2, the range of variations in soldercomposition between pre-coated solder layers is narrow, the timerequired until each solder is precipitated is relatively short, thicksolder layers can be formed, and the number of solder bridges found ineach pre-coated solder layer is small. On the other hand, with respectto the solder-precipitating composition of the comparative example 1,the range of variations in solder composition between pre-coated solderlayers is wide, the time required until each solder layer isprecipitated is relatively long, and thick solder layers cannot beformed, although the number of solder bridges found in each pre-coatedsolder layer is small.

EXAMPLES 3-5, COMPARATIVE EXAMPLE 2

To more clarify the advantage of the invention, further examinationswere carried out as regards variations in solder composition betweenpre-coated solder layers, the time required until each solder wasprecipitated, and the thickness of each pre-coated solder layer.

First, each of the solder-precipitating compositions of examples 3-5 anda comparative example 2 was prepared by mixing a corresponding alloypowder or Sn powder, a corresponding organic acid salt of Pb and acorresponding flux A (as used in the case of the example 1), which havecompositions as shown in table 4.

                  TABLE 4                                                         ______________________________________                                                                            COMPARA-                                                                      TIVE                                                EX-      EX-       EX-    EX-                                                 AMPLE    AMPLE     AMPLE  AMPLE                                               3        4         5      2                                         ______________________________________                                        SOLDER    35 *6    41 *7     60 *8                                            POWDER                                                                        Sn POWDER                           25                                        ORGANIC   40       30        20     45                                        ACID SALT                                                                     OF Pb                                                                         (Pb 24%)                                                                      FLUX A    25       29        20     30                                        ______________________________________                                         *6: Sn/Pb = 90/10                                                             *7: Sn/Pb = 80/20                                                             *8: Sn/Pb = 70/30                                                        

With respect to the solder-precipitating compositions of the examples3-5 and the comparative example 2, variations in solder compositionbetween pre-coated solder layers, the time required until each solderlayer was precipitated, and the thickness of each solder layer wereexamined. The variations in solder composition, the time required untileach solder layer was precipitated, and the thickness of each solderlayer are shown in table 5, the attached figure, and table 6,respectively. The variations were examined in a manner similar to theexamples 1 and 2. The required time was measured by subjecting the Sncontent to atomic absorption analysis after 30-second heating, 60-secondheating, 90-second heating and 120-second heating, respectively. Asregards the thickness, the thicknesses of solder layers obtained fromsolder-precipitating composition layers having thicknesses of 200 μm,300 μm, 400 μm and 500 μm and formed on 80-pin QFP patterns (the pitchof the pins is 0.8 mm), respectively, were measured. The estimation ofthe other points was performed in a manner similar to the case of theexamples 1 and 2.

                  TABLE 5                                                         ______________________________________                                                                                COM-                                                                          PARA-                                                                         TIVE                                                   EX-      EX-    EX-    EX-                                                    AMPLE    AMPLE  AMPLE  AMPLE                                                  3        4      5      2                                     ______________________________________                                        Sn    MINIMUM    61.9     61.8   61.8   62.0                                  CON-  VALUE                                                                   TENT  MAXIMUM    67.1     65.3   64.2   70.2                                        VALUE                                                                         DIFFER-    5.2      3.5    2.4    8.2                                         ENCE                                                                    ______________________________________                                        Target Composition is 63.0% (UNIT: wt %)                                  

                  TABLE 6                                                         ______________________________________                                        THICKNESS                           COMPARA-                                  OF SOLDER-                          TIVE                                      PRECIPI-   EX-      EX-      EX-    EX-                                       TATING     AMPLE    AMPLE    AMPLE  AMPLE                                     COMPOSITION                                                                              3        4        5      2                                         ______________________________________                                        200        33       45       75     20                                        300        45       61       97     25                                        400        58       79       125    28                                        500        70       93       150    32                                        ______________________________________                                        (UNIT: μm, Average Value)                                              

As is evident from table 5, the range of variations in soldercomposition is narrow between the solder-precipitating compositionsaccording to the examples 3-5 of the invention, which contain respectivealloy powders. In particular, it should be noted that the nearercomposition to a target composition the solder powder of thesolder-precipitating composition layer has, the narrower the range ofthe variations is. On the other hand, variations in solder compositionis wide between the solder-precipitating compositions according to thecomparative example 2, which contain a single metal powder. Further, asis evident from the figure, each of the solder-precipitatingcompositions according to the invention, which contain respective alloypowders, reaches the target composition after a short heating time. Inparticular, the nearer composition to the target composition the solderpowder of the solder-precipitating composition layer has, the shorterthe required time. On the other hand, the solder-precipitatingcomposition which contain the single metal powder requires a long timeto reach the target composition.

Moreover, as is evident from table 6, the solder-precipitatingcompositions according to the invention, which contain alloy powders,can have thick solder layers. In particular, it should be noted that thenearer composition to the target composition the solder powder of thesolder-precipitating composition layer has, the thicker the solder layeris. On the other hand, the solder-precipitating composition whichcontain the single metal powder can have only a thin solder layer.Further, table 6 indicates that the thicker the solder-precipitatingcomposition layer of the invention, the thicker the solder layer, andindicates that the solder-precipitating composition which contain thesingle metal powder cannot have a thick solder layer even if thesolder-precipitating composition layer is thick. This means that thereis a limit in the thickness of a solder layer formed from thesolder-precipitating composition which contain the single metal powder.

The above-described advantage results from the feature of the inventionthat the content of a substituted metal obtained by substitutionreaction can be reduced in a solder-precipitating composition whichcontains an alloy powder, and that the content of effective metalcomponents can be increased in the solder-precipitating composition.

EXAMPLE 6

To define an optimal range of the Pb content of the solder-precipitatingcomposition of the invention, the following experiments were carriedout:

First, a solder-precipitating composition according to the invention wasprepared by mixing 40.0 wt % solder powder (Sn/Pb=65/35) of an averageparticle size of 10 μm, 7.5 wt % Pb naphthenate as an organic acid saltof Pb, a 25.0 wt % rosin as an adhesive, 8.5 wt % triethanolamine as anactivator, and 19.5 wt % mineral spirit as a solvent.

EXAMPLES 7 AND 8, COMPARATIVE EXAMPLES 3 AND 4

Solder-depositing compositions of examples 7 and 8 and comparativeexamples 3 and 4 were prepared in a manner similar to the case of theexample 6 except that they contain different amounts of Pb naphthenateas shown in table 7.

EXAMPLE 9

A solder-precipitating composition of an example 9 was prepared in amanner similar to the case of the example 6, except that the content ofthe solder powder (Sn/Pb=65/35) is 20.0 wt % and the content of Pbnaphthenate is 20.0 wt % as shown in table 1.

The layers of the solder-precipitating compositions of the examples 6-9and the comparative examples 3 and 4 were formed, by screen printing, oncircuit boards having 160-pin QFP patterns (the pitch of the pins is 0.3mm), respectively, such that they have a thickness of 300 μm, and wereheated at 220° C. to deposit solder. These boards were cleaned bytoluene and dried, thereby forming solder layers on the QFP patterns ofthe boards.

Subsequently, solder bridges on the QFP patterns of the solder layersformed using the above-described solder-precipitating compositions wereobserved by the eyes. The number of the solder bridges are also shown intable 7.

Further, the observation results are shown in table 8.

                  TABLE 7                                                         ______________________________________                                                  CONTENT                  NUMBER                                               OF Pb         CONTENT    OF                                                   NAPHTHENATE   OF Pb      BRIDGES                                              (5) (Pb24%)   (%)        *9                                         ______________________________________                                        EXAMPLE   7.5           1.5        2-12                                       EXAMPLE   10.0          2.0        0-1                                        7                                                                             EXAMPLE   25.0          5.0        0                                          8                                                                             EXAMPLE   50.0          10.0       0                                          9                                                                             COMPARA-  0             0          13-18                                      TIVE                                                                          EXAMPLE                                                                       3                                                                             COMPARA-  5.0           1.0        17-20                                      TIVE                                                                          EXAMPLE                                                                       4                                                                             ______________________________________                                         *9 Number of Bridges in 160PIN QFP                                       

                  TABLE 8                                                         ______________________________________                                        EXAMPLE 6   The aggregation of the alloy powder                                           was restrained and a small of solder                                          bridges were found.                                               EXAMPLE 7   The aggregation of the alloy powder                                           was significantly restrained and                                              almost no solder bridges were found.                              EXAMPLE 8   The aggregation of the alloy powder                                           was restrained and no solder bridges                                          were found.                                                       EXAMPLE 9   The aggregation of the alloy powder                                           was restrained and no solder bridges                                          were found.                                                       COMPARA-    A metal powder in the remainder was                               TIVE        all aggregated on pads, and triple                                EXAMPLE 3   bridges, etc. were found in many                                              places.                                                           COMPARA-    A metal powder in the remainder was                               TIVE        all aggregated on pads, and triple                                EXAMPLE 4   bridges, etc. were found in many                                              places.                                                           ______________________________________                                    

As is evident from tables 7 and 8, where Pb naphthenate is used as anorganic acid salt of Pb, occurrence of solder bridges can be preventedat the time of pre-coating a fine pattern with solder if the Pb contentof the solder-precipitating composition is 1.5 wt % or more. Thus, inaccordance with the composition of an alloy powder and the kind of anorganic acid salt of Pb, the Pb content has a range which enablesoccurrence of solder bridges, which may occur at the time of pre-coatinga fine pattern with solder, to be effectively prevented.

Thereafter, with respect to the solder-precipitating compositions of theexamples 6-9, examinations were carried out as regards variations insolder composition between pre-coated solder layers, the time (treatmenttime) required until each solder was precipitated, and the thickness ofeach pre-coated solder layer. As a result, it was found thatsolder-precipitating compositions according to each of the examples 6-9show a narrow range of variations in solder composition, require a shorttime until each solder layer is precipitated, and provide thick solderlayers.

EXAMPLES 10 AND 11

Solder-precipitating compositions of examples 10 and 11 were prepared ina manner similar to the example 6, except that Pb stearinate and Pbacetate were used instead of Pb naphthenate in the examples 10 and 11,respectively. The contents of Pb stearinate and Pb acetate were adjustedsuch that the Pb content of each solder-precipitating composition was2.0 wt %.

Each of the solder-precipitating compositions of the examples 10 and 11was formed, by screen printing, on a 160-pin QFP pattern (the pitch ofthe pins is 0.3 mm) such that the it forms a layer of 300 μm. Thesolder-precipitating composition layer is heated at 220° C. to depositsolder. The resultant structure is cleaned using toluene and dried,thereby forming a solder layer.

Subsequently, with respect to solder layers formed on boards, using eachsolder-precipitating composition, examinations were carried out asregards variations in solder composition between pre-coated solderlayers, the time (treatment time) required until each solder wasprecipitated, and the thickness of each pre-coated solder layer. As aresult, it was found that solder-precipitating compositions according toeach of the examples 10 and 11 show a narrow range of variations insolder composition, require a short time until each solder layer isprecipitated, and provide thick solder layers each having no solderbridges.

Since as explained above, a solder-precipitating composition accordingto the invention contains Sn-Pb alloy powder and an organic acid salt ofPb as essential components, it can provide, on a pad in a short time, asolder layer with a uniform and sufficient thickness, and cap performpre-coating of solder without forming a solder bridge even if pads arearranged at narrow intervals.

We claim:
 1. A solder-precipitating composition containing Sn-Pb alloypowder and an organic acid salt of Pb as essential components, andcapable of precipitating solder as a result of substitution reactionbetween Sn contained in the Sn-Pb alloy powder and Pb ions contained inthe organic acid salt of Pb.
 2. The solder-precipitating compositionaccording to claim 1, further containing a solvent and aviscosity-holding agent.
 3. The solder-precipitating compositionaccording to claim 1, wherein the content of the Sn-Pb alloy powder is20-70 wt %, and the content of the organic acid salt of Pb is 7.5-50 wt%.
 4. The solder-precipitating composition according to claim 1, whereinthe precipitated solder has a eutectic structure of Sn and Pb, the Sn-Pballoy powder contains 65-90 wt % Sn, and the content of the Sn-Pb alloypowder is 20-70 wt % of the solder-precipitating composition.
 5. Thesolder-precipitating composition according to claim 1, wherein the Sn-Pballoy powder has a particle size of 1-20 μm.
 6. A solder-precipitatingmethod comprising the steps of:supplying a solder-precipitatingcomposition which contains Sn-Pb alloy powder and an organic acid saltof Pb as essential components; and heating the solder-precipitatingcomposition to precipitate solder as a result of substitution reactionbetween Sn in the Sn-Pb alloy powder and Pb ions in the organic acidsalt of Pb.
 7. The method according to claim 6, wherein the heatingtemperature is 210°-220° C.
 8. A mounting method comprising the stepsof:supplying a conductor on a board with a solder-precipitatingcomposition which contains Sn-Pb alloy powder and an organic acid saltof Pb as essential components; heating the solder-precipitatingcomposition supplied on the conductor, to precipitate solder on theconductor and pre-coat the conductor with solder, as a result of ionsubstitution reaction between Sn in the Sn-Pb alloy powder and Pb ionsin the organic acid salt of Pb; mounting a device on the solderpre-coated conductor; and melting the solder layer to securely mount thedevice on the conductor.
 9. The method according to claim 8, wherein theheating temperature for precipitating solder is 210°-220° C.